I think that you have a very interesting thesis statement. I would spend some time in talking about how Proctor has to assume the moral position of being on the "right side of history" because so much of Salem is on the wrong side. Proctor is an ordinary man in times that are pleasing for extraordinary individuals. I think that you could stress that Proctor's decision to assmue moral responsiblity is fostered by the fact that there is so much hypocrisy and ambivalence in Salem throughout the novel. Proctor becomes incensed that the townspeople are so filled with fear to speak out, and those who are "filled with pasisonate intensity," to quote Yeats, are the ones who are in the position of power. It is this disgust and sense of revulsion that help to incite him to take action. I think that you can trace the development of how Salem descends into madness and chaos as evidence of how Proctor sees that his choice is the only choice that can be made in such a context.

I think you could also take this from the perspective that Proctor is doing the right thing in terms of his beliefs in God. He is not lying about his predicament on earth, but rather is telling the truth, knowing that the reward is coming to him in heaven. If you look at it from that perspective, you can get into the strength that his faith provides him and how much it means to him throughout. That strength comes in the form of his beliefs. Most of the other people of Salem have confused themselves about their beliefs, but Proctor hasn't. He takes strength in that. He falters, but ultimately makes the right decision. I think he also feels a sense of leadership toward the others. He is the most upstanding/strong/powerful person in the group that is hanged at the end. He is the one with the most to lose upon his death. Yet he still does the right thing.